This invention relates generally to data buses and particularly to taps for carrying signals to and from data buses. Still more particularly, this invention relates to taps for carrying signals to and from fiber optic data buses.
Optical fiber provides significant advantages over other wave guide means in the amount of information that may be transmitted. Since optical signals guided by optical fiber have frequencies much greater than other guided waves, optical fiber provides more frequency bands, or channels, in which a carrier signal may be modulated to transmit information. Light signals carried by optical fiber are less susceptible to environmental interferences than electromagnetic signals of lower frequency carried by wires or other types of waveguides.
Difficulty arises in constructing a practical multistation data bus that could be accessed by a multiplicity of users because of the relatively high losses of fiber optic couplers required to couple signals to and from the data bus. Prior fiber optic data buses have the disadvantage of providing only one way transmission of data and therefore require a complete loop of optical fiber in order to form an interactive network between user stations or between systems of sensors and control devices connected to such data buses.
Some familiarity with the propagation characteristics of light within an optical fiber will facilitate an understanding of both the present invention and the prior art. Therefore, a brief discussion of fiber optic waveguides, normal modes of propagation of light in such waveguides and polarization of light is presented.
The behavior of an optical wave at an interface between two dielectric materials depends upon the refractive indices of the two materials. If the refractive indices of the two dielectrics are identical, then the wave propagates across the interface without experiencing any change. In the general case of different refractive indices, however, there will be a reflected wave, which remains in the medium in which the wave was first propagating, and a refracted wave, which propagates beyond the dielectric interface into the second material. The relative intensities of the reflected and refracted waves depend upon the angle of incidence and the difference between the refractive indices of the two materials. If an optical wave originally propagating in the higher index material strikes the interface at an angle of incidence greater than or equal to a critical angle, there will be no refracted wave propagated across the interface; and essentially all of the wave will be totally internally reflected back into the high index region. An exponentially decaying evanescent wave extends a small distance beyond the interface.
Most optical fiber has an elongated generally cylindrical core of higher refractive index and a cladding of lower refractive index surrounding the core. Optical fibers use the principle of total internal reflection to confine the energy associated with an optical wave to the core. The diameter of the core is so small that a light beam propagating in the core strikes the core only at angles greater than the critical angle. Therefore, a light beam follows an essentially zig-zag path in the core as it moves between points on the core-cladding interface.
It is well-known that a light wave may be represented by a timevarying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light waave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions ar directly related to the distribution of energy within the structure.
The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate unattenuated are the normal modes. A single mode fiber will guide only one energy distribution, and a multimode fiber will simultaneously guide a plurality of energy distributions. The primary characteristic that determines whether fiber is single mode or multimode is the ratio of the diameter of the fiber core to the wavelength of the light propagated by the fiber.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. If only the electric field vector is perpendicular to the direction of propagation, which is usually called the optic axis, then the wave is said to be transverse electric (TE) mode. If only the magnetic field vector is perpendicular to to the optic axis, the wave is a transverse magnetic (TM) mode. If both the electric and magnetic field vectors are perpendicular to the optic axis, then the wave is a transverse electromagnetic (TEM) mode. None of the normal modes require a definite direction of the field components; and in a TE mode, for example, the electric field may be in any direction that is perpendicular to the optic axis.
The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for each mode. If all the electric field vectors in a wave point in only one particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude and 45.degree. out of phase, the electric field is circularly polarized because the net electric field is then a vector that rotates around the optic axis at an angular velocity equal to the frequency of the wave. If the two linear polarizations have unequal magnitudes and phases that are neither equal nor opposite, the wave has elliptical polarization. In general, any arbitrary polarization can be represented by either the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two oppositely directed elliptical having orthogonal semi-major axes.
Propagation characteristics, velocity, for example, of an optical signal depend upon the index of refraction of the medium through which the light propagates. Certain materials have different refractive indices for different polarizations. A material that has two refractive indices is said to e birefringent.
Fiber optic sensors have geophysical applications for detecting acoustic signals generated in geophysical research. Fiber optic sensors find applications in other situations for detecting acoustic vibrations. Such applications may require many fiber optic sensors. In order to eliminate the necessity of having a separate strand of fiber optic material for each sensor, it is desirable to connect many separate sensors to a single data bus also formed of optical fiber. These connections are achieved with the use of fiber optic couplers.
There are four general classes of multimode couplers that have been cnsidered for constructing a multistation fiber optic data bus: (1) fused biconical couplers, (2) bulk optical couplers, (3) evanescent field couplers and (4) direct core intercept couplers. The basic structures and methods for forming these couplers are well known in the fiber optics art.
The fused biconical tapered coupler is fabricated by twisting together two fibers over a short length of one to three centimeters and then heating the twisted region together while holding the fibers under tension. The fibers partially fuse because of the heat applied thereto. The stretching process causes the fibers to taper symmetrically on each side of the heated area, which gives rise to the descriptive term "biconical".
As the fiber cross section is reduced, modes guided by the cores expand into the cladding of both fibers. This mode overlap is the source of coupling. Light ordinarily will not escape from the cladding into the surrounding environment because the index of refraction of the cladding is higher than that of air. The core has a higher refractive index than the cladding; therefore, light will cross the core-cladding interface into the core. Since the core has a higher refractive index than the cladding, most of the light previously in the cladding ordinarily will remain guided by the core.
Since the fused biconical tapered coupler depends upon mode expansion into the weakly guiding cladding area of the fibers and subsequent compression back into the normal guided core modes after the coupling region has been traversed, these couplers have the disadvantage of having high radiation losses. Such losses render this type of coupler unsuitable for forming a data bus having a great number of taps because the signal losses would be so great that no signal could be distinguished from the noise inherent in the system.
Fused biconical tapered couplers are also sensitive to the material surrounding the tapered portions of the fibers. Potting compounds exhibiting appropriate index of refraction and index stability in the presence of temperature changes are difficult to obtain. Therefore, a fully encapsulated fused biconical tapered multimode coupler generally has an insertion loss of a minimum of about 0.5 dB.
There are numerous techniques for fabricating bulk optics couplers using miniaturized bulk optics beam splitter having dimensions approaching those of optical fibers. Diffraction gratings and dichroic filters are also employed to provide coupling in devices such as wave length division multiplexers. All bulk optics couplers have the problems of high loss and high production cost, which render bulk optics couplers unsuitable for forming a multistation data bus.
Evanescent field couplers provide coupling by means of overlap of the exponentially decaying evanescent fields that surround the modes guided by the fiber cores. In single mode fiber, a significant amplitude of the evanescent field extends several microns radially outward from the core into the cladding. If most of the cladding is removed from two fibers and the resulting nearly exposed cores placed close together, the evanescent field of light guided by one fiber overlaps into the core of the adjacent fiber; and coupling occurs.
The amount of power coupled from one fiber to the other is a function of the core separation, distance over which the cores are in proximity and the mode propagation characteristics of the cores. In multimode fibers only those modes having a relatively high fraction of the energy distributed near the core/cladding boundary have evanescent fields that extend into the cladding sufficiently to cause coupling. These higher order modes carry only a small portion of the total transmitted power. Therefore, it is necessary to bring the cores of both fibers into very close proximity over a large distance to achieve any significant coupling. In practical devices, the cores must be in physical contact over a distance of one to three centimetrs. Maintaining this relation between two fiber cores to fractions of a micron under environmental conditions is extremely difficult.
The direct core intercept coupler does not employ evanescent coupling, but rather, merges the cores of the coupled fibers over relatively short interaction distances of about one to six millimeters. The coupling is proportional to the core areas intercepted and permits bonding of the interaction region 76 between fibers by fusion welding or adhesives. Previously, fully potted direct core intercept couplers, like the other couplers described above, have exhibited losses too high for forming a multiple tap data bus. It has also been difficult to maintain the fraction of enery coupled by such couplers to within specified limits satisfactory for forming a multiple tap data bus.
Still another difficulty with previous direct core intercept couplers has been the cost and time required for their manufacture. Production of a fiber optic multiport data bus at reasonable cost requires low cost, easily made couplers which will retain a specified coupling ratio when potted and exposed to environmental disturbances such as temperature fluctuations and vibrations.